A vehicle may include an electronic key system that verifies a wireless signal transmitted from an electronic key to control the locking and unlocking of the vehicle doors and the starting of a drive source. An antenna device that receives wireless signals is arranged in such a vehicle that includes the electronic key system. Japanese Laid-Open Patent Publication No. 2012-29137 describes a reverse-L-shaped antenna fixed to a substrate. The reverse-L-shaped antenna includes a plate-like leg, which extends orthogonal to the upper surface of the substrate, and a plate-like arm, which is continuous with a distal portion of the leg and which extends parallel to the upper surface of the substrate.
The antenna device is often installed in a vehicle door. The vehicle door vibrates when the vehicle travels and when the door opens and closes. The vibration is transmitted from the leg to the arm in the antenna. As described in the publication, the conventional antenna (arm) is formed to have a uniform thickness in one thickness-wise direction. As illustrated in the schematic view of FIG. 5, in the conventional structure, a plate-like antenna 51 is cantilevered by a substrate 52. Thus, the antenna 51 has a tendency to vibrate in the thickness-wise direction. In such a structure, vibration of the vehicle door may cause resonance at the arm of the antenna 51. Consequently, vibrational noise resulting from the resonance of the antenna 51 may be audible to a vehicle occupant.